So, uh, this happened.
And it's all this little guy's fault.
A few weeks ago I was messing around on my mage Glasya, who's just on storage bank duty right now. And - I really don't know why, since I wasn't really playing her or anything - I decided to conjure up a little mage familiar.
I think maybe it was just to see if Presence of Mind works on the Conjure Familiar spellcast? (It does.) Regardless, I cast it, and...nothing happened.
Turns out my bags were full, so the spell failed. I shrugged and went on with my life, since both spells were now on cooldown and I wasn't going to wait around.
Fast forward to a few days later. I log back onto Glasya to fetch a transmog piece, and much to my surprise, what do I find but an Arcane Familiar Stone sitting in my mailbox, courtesy of the Postmaster!
I found this mildly interesting, because usually when you try to create an item when your bags are full - such as making a flask or cutting a gem - you just get an error message and the creation is canceled. (Trust me, I know this VERY WELL.) But for some reason, conjuring a familiar with your bags full just shunts the familiar immediately off to the mailbox.
Naturally, I had to test this further...
So where did that error message come from? Well, apparently the game really doesn't like it when you have a familiar stone in your bags and try to obtain another. Normally, if you have a stone already and try to cast the spell again, you just get a "You have too many of that item already" error message.
But if you try to loot a second stone from your mailbox - somewhere a familiar stone should never be found - the game freaks out and tells you that you can "only" carry 1,066,532,864 at one time.
Is this useful in any way? Nah, not at all. But I find it kind of funny to imagine this invisible NPC Postmaster who is already so busy retrieving all the misplaced LFR/Flex loot, satchels, and those stupid Rimes of the Time Lost Mariner for those of us with perpetually full bags. And how pissed off they must be to now have to also handle FAMILIAR STONES, of all things. Poor Postmasters.